This invention relates to a wireless receiving subsystem. More particularly, this invention pertains to a wireless receiving subsystem with an antenna connected to a signal input of a broadcast receiver such as a television or a radio.
An ordinary radio or television receiving subsystem consists of an antenna, a transmission line connecting the antenna to the radio or television set and the first stage of the set's receiver, often referred to as the front end. The antenna will usually be designed with an instantaneous bandwidth equal to the tunable bandwidth of the receiver.
The antenna receives electromagnetic energy in its operating band and sends it to the set via a transmission line. The received energy consists of a blend of the desired signals transmitted by the broadcast stations and a lot of other undesirable ingredients variously called noise or interference depending on the source and spectral composition of each contributor. In a television picture, what is called interference will usually result in some sort of undesirable pattern of dots or lines or squiggly lines moving about the screen or some segment of it. Interference is generated by equipment of some sort; examples are engine ignitions, radio or television broadcasts other than the one we want, elevators, diathermy, machine shops, motors, etc. Antennas can be endowed with one or more deep nulls, narrow angular regions of very low sensitivity, which can be directed at strong sources of interference. A special case and the most familiar example of interference is that caused by the desired signal arriving at the antenna by two or more paths of different length, resulting in "ghosting." Noise results in "snow," a random distribution of fuzziness or fuzzy dots throughout the picture. Noise comes from all directions at all frequencies and, for most purposes, the noise power received by a consumer antenna cannot be diminished by any means. Some kinds of interference are difficult to distinguish from pure noise and their effects are usefully considered together with the effects of noise.
The measure of adequacy of desired signal reception with respect to noise power is called the signal-to-noise ratio ("SNR"). For a particular signal bandwidth, this measure is just what it says, the total desired signal power divided by the total noise power. The problematic part of the noise issue is that the ratio of signal to noise at the antenna terminals is degraded--usually by a slight amount--in its passage down the transmission line and the ratio of signal to noise delivered to the radio/television set's terminals is degraded by the first stage of the receiver. For many applications, the receiver is the dominant source of noise power. Much effort has been directed at achieving "low noise receivers" and "low noise amplifiers" to improve reception quality.
Modern receiving sets feature very good first stage amplifiers which amplify incoming signals (including received signal power+received noise power+transmission line noise power) delivered by the transmission line while adding some amplifier noise power. Subsequent stages of amplification also add noise power, but the parameters of the first stage of amplification almost always dominate these considerations. Many advances in picture quality in recent decades are the result of improvements in the front end amplifier designs.
In the consumer marketplace, there has been a trend toward providing an amplifier located at the antenna, often integrated into the antenna enclosure. At first glance, this appears to be a reasonable thing to do. The signal at the antenna is amplified so the cable losses and the noise power contributions of the cable are relatively less important.
However, there are several features that may be overlooked in this simplistic assessment: every operation adds noise power and the addition of an antenna amplifier results in degraded SNR at the amplifier output compared with that at the antenna terminals; the antenna amplifier is unlikely to offer noise performance as good as a modern radio or television set's first stage amplifier; an antenna amplifier for television will often have a pass band of about 50 to 850 MHZ and it is quite possible for the totality of signals received and amplified in that band to be powerful enough to saturate (overdrive) some part of the receiving chain, with attendant sound or picture distortions. This last factor can result in, for example, an overflying aircraft transmitting at 125 MHZ distorting the quality of signals received from any television or FM broadcast. Similarly, a broadcast station located very close to a receiving site could produce distortions at every broadcast station frequency. Without the additional (antenna) amplifier, signal levels are reduced and saturation is less likely.
On the other hand, when a long cable run connects the antenna with the radio or television set, an amplifier at the antenna may enhance the SNR delivered to it. For a hundred foot length of RG-6 coaxial cable, for example, attenuation in the UHF television band exceeds 71/2 dB and an amplifier at the antenna end of the cable will probably be useful. At the low VHF television band and at the FM radio band, the same cable results in attenuations ranging from 11/2 to 3 dB and an antenna amplifier will more likely increase the system SNR than diminish it. At the high VHF television band, attenuations of 4 to 41/2 dB are obtained and SNR might improve ever so slightly (try it and see is the best thing to do).
Also, where the broadcast receiver is a very old radio or television set and the antenna amplifier is state of the art, results will be better with the amplifier than without it.
An outdoor TV antenna product exists with an integrated amplifier. DC power is provided to the antenna amplifier through the coaxial cable which connects to the TV set through a small "power injector" unit which plugs into a wall outlet and has imperceptible attenuation of RF signals. This product exhibits the disadvantages discussed above. With 100 feet of cable, the amplifier helps at the higher part of the UHF band and makes little difference otherwise. With 6 feet of cable, the amplifier hurts a few channels and does not affect the others noticeably. An especially significant disadvantage of this existing product is the large signal attenuation obtained when the power injector is unplugged. This results in excellent pictures literally disappearing when power is removed.